There have been conventionally known engine cooling devices so configured as to circulate cooling water through an internal combustion engine (engine) that outputs a driving force for vehicle running, thereby to cool the engine. In general, cooling water circulated through this type of engine cooling devices is utilized as a heat source for heating air (fluid to be heated) sent into a vehicle compartment, in vehicle air conditioners.
For example, in the engine cooling device for vehicles described in Patent Document 1 (U.S. Pat. No. 5,337,704), an engine is provided therein with a head-side flow path circulating cooling water for cooling the cylinder head and a block-side flow path circulating cooling water for cooling the cylinder block, and cooling water flowing out of the head-side flow path is utilized as a heat source for heating air.
With respect to engines mounted in a vehicle, there is a demand that their output is increased without increasing their physical size. As a means for meeting this demand, there are known, for example, superchargers that supercharges air for fuel combustion (intake air) sucked into an engine and the like. In an engine equipped with a supercharger, however, the compression ratio in the combustion chamber is increased with increase in boost pressure and this makes knocking prone to occur.
Thus, in the engine equipped with a supercharger, the antiknock performance is enhanced by such a means as lowering the temperature of a combustion chamber as compared with an engine without a supercharger. Therefore, when a supercharger is equipped in the engine in Patent Document 1, it is necessary to lower the temperature of the combustion chamber by such a means as increasing the flow amount of cooling water circulated through the head-side flow path.
However, this involves a problem. If the flow amount of cooling water circulated through the head-side flow path is increased to lower the temperature of the combustion chamber, the temperature of cooling water flowing out of the head-side flow path is also lowered. That is, the temperature of cooling water utilized as a heat source for heating air is also lowered. As a result, the temperature of the air cannot be sufficiently raised and there is a possibility that the air in the vehicle compartment cannot be appropriately conditioned (especially, heated).